1. Field of the Invention
The present invention relates to a method for loudness calibration of a multi-channel sound systems: The present invention also relates to a multichannel sound system.
2. Description of Related Art
The following terminology is used in the document. The reproduction level of a sound system is controlled by volume control, which changes the channel gains equally. The channel gain is a channel specific control with respect to initial level to be used for compensating various differences between loud-speakers e.g. in sensitivity. The level calibration is used to adjust the channel gains to give equal physical measure at the listening position using a test signal. The loudness calibration is used to adjust the channel gains to give equal loudness at the listening position using test signal. The loudness is an auditory sensation and as such it can not be directly measured. It depends on acoustical intensity, frequency, duration and spectral complexity. These are physical attributes that can be measured and the loudness can be estimated from those using existing models [3,4,5].
Domestic multichannel sound systems, with or without pictures, are becoming increasingly popular. A sound system has to be calibrated to ensure the best possible aural environment. A traditional stereo system usually has two identical loudspeakers. When they are set-up symmetrically in a room and the listener stays with equal distance to both of them, the level calibration is quite simple. The system is provided with balance control, which can be set to middle; equal gains to both channels. If the listening position is closer to one of the loudspeakers or the loudspeakers are set-up asymmetrically to the room, the balance must be re-adjusted. This provides the listener with a means of level control.
The current trend in the field of domestic sound system is towards multichannel systems having more that two loudspeakers, like the 5 channel system shown in FIG. 1a. With multichannel system the calibration situation can be far more complex than with traditional stereo system. The loudspeakers often have different characteristics; they differ in bandwidth, sensitivity, directivity etc. Furthermore the positioning of a loudspeaker has a great effect on room coupling. The loudspeaker in a corner of the room or just close to one wall may have very different amplitude response characteristics than one located away from the walls.
In the ideal situation such as specified in e.g. ITU-R BS.775-1, shown in FIG. 1a, the central loudspeaker 102, the left and right loudspeakers 104a and 103a as well as left and right surround loudspeakers 105a and 106a have an equal distance to the listening position 101. In FIG. 1b a more realistic loudspeaker placement is shown. The loudspeakers 102, 103a, 104a, 105a, 106a are normally placed near the walls. When the shape of the room 110b is not ideal from the viewpoint of aural environment, it is typical that the distances from the loudspeakers 102, 103a, 104a, 105a, 106a to the listening location 101 are not equal. With these circumstances even matching the reproduction level of centre channel from the loudspeaker 102 to usually identical left and right channels from the loudspeakers 104a and 103a is difficult. And further the situation with surround channels from loudspeakers 106a and 105a is even more problematic. The situation becomes even more problematic when the room coupling effects are taken into account. These problems relate to bandwidth, sensitivity, directivity, and distances of the loudspeakers and room interaction.
The object of the sound system calibration is to calibrate the loudspeakers 102, 103a, 104a, 105a and 106a so that in the listening position 101 it seems, or rather sounds, like the sound is coming from the virtual loudspeakers 103b, 104b, 105b and 106b, all at equal distances from a listening position 101. This sensation of virtual loudspeakers is achieved mainly by the two methods. First, by changing delay times of each loudspeaker 102, 103a, 104a, 105a, 106a so that sound meant to be heard simultaneously are transmitted at different times by each loudspeaker so that the sounds arrive to the listening position 101 simultaneously. Secondly, by adjusting the gain of each loudspeaker so that they produce equal loudness at the listening position 101.
There are basically two methods for calibrating a multichannel sound system. The calibration can either be done automatically without human perception or subjectively when the person calibrating the system calibrates the system according his personal subjective audio perceptions.
An automatic calibration is quite an accurate method for calibrating delay times for each loudspeaker, but not as good for loudness calibration. The loudness is a auditory sensation, and as such it cannot be directly measured in the same manner as acoustic pressure or intensity, which are physical attributes and as such straightforward to measure. Therefore a subjective calibration is mainly used for loudness calibration. So called xe2x80x9cpink noisexe2x80x9d [1] is most often used as a test signal in subjective calibration, because its spectrum correlates well to statistical properties of natural sound. Bandlimited test sounds are normally used in subjective loudness calibration, to avoid problems with room coupling on lower frequencies and location sensitivity with the higher frequencies.
In FIG. 2 a flow chart of the prior art method 200 for automatic sound system calibration is shown. In step 201 a test signal is generated. The test signal is preferably some pseudorandom signal allowing the calculation of the periodic impulse response of the aural environment under study. Said aural environmental includes the actual multichannel sound system as well as loudspeakers and the listening space as they give a considerable contribution to the aural environment. One possible test signal type is a maximum-length sequence (MLS) [2].
In the step 202 the test signal is transmitted via a sound source i.e. loudspeaker to the listening space. In the step 203 the test signal is received by a microphone at the preferred listening position.
In step 204 a cross correlation between the original signal generated in step 201 and the signal received in step 203 is carried out. If the test signal is an MLS or similar signal, this gives in step 205 the periodic impulse response of the aural environmental. In step 207 various parameters giving information about aural properties the aural environment in the time domain, like arrival times, early reflection and room reverberation information are calculated from the periodic impulse response.
In step 206 the periodic impulse response of the system is transformed to the frequency domain using a fast fourier transform (FFT) algorithm. In step 208 various frequency domain properties of the aural environment, like phase and amplitude response, are calculated from FFT transform of the periodic impulse response.
In step 209 an automatic calibration is carried out according to the time and frequency domain information calculated in steps 207 and 208. By applying similar calibration for each sound source, the whole system can be calibrated.
The problem of the above stated prior art is that with automatic calibration the achieved calibration is not sufficiently good due the subjective nature of the loudness. The calibration according only to physical terms does not necessarily provide optimum calibration in perceptual terms. On the other hand, when using subjective loudness calibration the test signals do not excite the room or the listener to the extent the programme material does. In addition some frequency ranges are more dominant at the perceptual level, thus making the calibration based on only to these ranges. Therefore the calibration according to the prior art does not give sufficiently accurate calibration causing the spatial attributes produced by the system to be different from the intentions of the programme maker.
In the prior art different test signals are used in automated and subjective calibration, thus making the calibration procedure and systems unnecessary complex.
An object of the present invention is to provide a new method and a new multichannel sound system for carrying out the loudness calibration, so that accurate subjective calibration can be achieved on a wider frequency range compared to the prior art, thus making the loudness calibration of the multichannel sound system more accurate.
Further the object of the present invention is to provide a new method and a new multichannel sound system for carrying out both subjective and objective calibration using the same test signal in both calibrations. Therefore the calibration phase of the sound system can be simplified.
The above stated objects are achieved by psychoacoustically shaping the test signal. The psychoacoustically shaped test signal preferably is a pseudorandom test signal suitable for both automatic and subjective loudness calibration. Further the psychoacoustically shaped test signal has preferably essentially constant specific loudness on the frequency range essential for aural perception.
Compared to the prior art, the present invention gives significant advantages. Using the method and the system according the invention one can achieve more accurate loudness calibration using simpler and easier procedures compared to the prior art.